The present invention relates to optical scanners in general and more particularly to optical scanners employing grooved grease bearings.
Laser printers use a rotating polygon mirror optical scanner to produce the latent image on the photoreceptor. The polygon mirror is typically supported by a rotor, which is driven at a high angular velocity by an electric motor. The motor/polygon mirror assembly used in high volume/high resolution printers must operate at speeds from 15,000 to 35,000 RPM for 15,000 hours, produce little audible noise, be contamination free and survive occasional shock loading.
Conventional ball bearings can not survive at the high speeds of an optical scanner. They are extremely noisy, particularly in comparison to the quiet office environment that most optical scanners and laser printers operate in. Additionally, ball bearings at high speeds suffer from run-out, deviation from the desired spin axis, which is unacceptable for the delicate beam alignment needed for optical scanning.
Gas bearings are normally used in high speed optical scanners. They rely on the pressure created between a pair of cylindrical surfaces in relative motion to support the rotor load. One solid cylindrical surface, typically part of the rotating polygon mirror, is within a complementary stationary hollow cylindrical surface, typically part of the motor and scanner housing. One of the two cylindrical surfaces is usually grooved.
Since the gas in the gas bearing is typically air, which has a notoriously low viscosity, the clearance space between the rotating part of the bearing and the stationary part of the bearing is quite small, on the order of 3.5 microns (0.000 135 inch) with a latitude of +/-0.25 micron (0.000 010 inch). This small manufacturing latitude makes gas bearings both expensive to make and sensitive to contamination and moisture.
Furthermore, the lifetime of gas bearings is limited by the wear of the bearing surfaces rubbing together when the optical scanner is started and stopped. The cylindrical surfaces touch at rest and static friction must be overcome before the polygon mirror rotates. The friction and surface rubbing also limits the minimum rotational speed at which the gas bearings can be used. The wear, static friction and minimum speed problems are compounded when the rotor is horizontally oriented.
It is an object of this invention, therefore, to provide high-speed, long-life, reduced noise, shock-resistant bearings for optical scanners.
It is another object of this invention to provide an optical scanner which can operate in any angular orientation and at any rotational speed, is not effected by static friction and can be started and stopped without concern for the wear of the bearing surfaces.
It is another object of this invention to provide a high speed scanner with bearings that are simple and inexpensive to manufacture.